Veneer dryers are commonly used in the wood products industry to lower the moisture content of veneer for plywood manufacturing and other applications. Logs are peeled or sliced to produce sheets of “wet” veneer, which is then loaded into a veneer dryer for drying. If the veneer is not dried to a particular moisture content, further processing stages may be adversely affected. For example, if the veneer will be glued to another layer of wood, too much residual moisture may prevent proper adhesion. Conversely, veneer that is too dry may absorb more glue, resulting in higher processing costs.
Jet veneer dryers have become increasingly popular. Jet veneer dryers are designed to dry veneer by directing air to impinge on the veneer surface at high velocities, commonly 2,000-10,000 feet per minute. Veneer jet dryers may be either direct-fired or indirect-heated. In direct-fired dryers, hot combustion gases from a direct fired air heater are blended with recirculated air from the dryer to reduce the combustion gas temperature. The blended gases are then circulated over the veneer. In contrast, indirect-heated veneer dryers use steam or circulated hot oil coils to heat the recirculated air before it passes over the veneer.
A typical jet veneer dryer is an elongate chamber with an upstream drying section, a downstream cooling section, and 1-6 vertically stacked “decks” extending from the input end of the drying section to the output end of the cooling section. Within the drying section, each deck has upper and lower driven rollers or belts for conveying the sheets of veneer along a corresponding flow path through the drying section. Between adjacent pairs of veneer rollers or belt supporting rollers are positioned pairs of elongated vertically-opposed jet tubes that extend horizontally across the deck from an air-delivery side to a generally opposite air return side. The drying section also includes an air circulation system with air-delivery side plenum, a return side plenum, heaters, and fans.
The jet tubes are generally rectangular in cross-section and tapered longitudinally from a large open end at the air-delivery side to a small closed end with a mounting bracket at the return side. The large open end is usually sealed against the framework of the drying section by a plate or other such feature. A plurality of apertures are provided along one surface of the jet tubes. Some of the jet tubes are mounted below the veneer flow path in an aperture-side-up orientation, and others are mounted above the veneer flow path in an aperture-side-down orientation. Heated air moves under pressure from the air-delivery side plenum into the open end of the jet tubes, flowing through the holes in the veneer-facing surfaces of the jet tubes and against the veneer moving along the flow path.
Jet dryers have some disadvantages. One drawback is that the narrow ends of the jet tubes tend to become plugged with accumulated debris. The accumulated debris reduces the flow of heated air at the return side of the decks, and can also cause a fire within the drying section. Conventional jet dryers must be cleaned typically at three week intervals to remove condensed volatile organic compounds (VOC's) and wood debris. A common method of removing debris from jet tubes involve using a flood of water that back washes and floats the debris from the tube as it drains. Another method uses a water or air lance which is activated after the lance is inserted from the large end of the tube to the small end and then is slowly backed out removing the debris as the lance is removed. Some jet tubes have a small aperture at the narrow end and a movable or removable cap or plug fitted over the opening to allow air or water to be introduced through the opening in the narrow end. A fourth method is to remove each jet tube from the dryer, position the jet tube vertically with the tapered end up, and pound the open end of the jet tube against the floor to dislodge debris. These cleaning methods are time-consuming, labor-intensive, and costly, and can present a risk to personnel.